(CNN) -- Minnesota Sen. Amy Klobuchar -- a superdelegate holdout from a traditional swing state -- backed fellow freshman Sen. Barack Obama Monday morning.
While Obama rival Sen. Hillary Clinton began the presidential campaign with a massive advantage in Congressional endorsements, Klobuchar's endorsement now gives Obama a 13-11 edge over Clinton among their Democratic Senate colleagues.
"I believe I have an obligation to try to bring our party together," Klobuchar said, adding that continuing to stay silent would be, in the words of her 12-year-old daughter, "awkward, Mom. Awkward."
Clinton maintains a 243-212 advantage among superdelegates -- 800 or so elected officials and party activists who are free to cast their vote how they wish -- in the most recent CNN count.
But since Super Tuesday on February 5, Obama has added more than twice as many to his total as she has, 106-50. Since the March 4 contests, the trend is greater than 3-to-1.
Clinton's count would be greater -- but the party has not resolved the thorny question of how to seat the Florida and Michigan delegations. Currently, neither state will be allowed to send pledged or superdelegates to the summer convention.
The tide of recent superdelegate nods, coupled with increasingly blunt calls from some Obama supporters and undeclared party heavyweights for the race to end well before the August nominating convention, has led to a superdelegate campaign -- parallel to, and at this point as critical as, the primary race itself -- from the Clinton team.
Last week, a group of major Clinton donors -- who had donated roughly $24 million to the party over the past decade -- sent a message to House Speaker Nancy Pelosi, who had made public comments urging superdelegates to nominate the pledged delegate leader, calling on her to reconsider her position.
Pelosi has not endorsed a presidential candidate; neither have such high-profile party leaders as former Vice President Al Gore, former New York Gov. Mario Cuomo, Jesse Jackson and Senate Majority Leader Harry Reid of Nevada.
Obama maintains a slight lead over Clinton among pledged delegates, which are awarded based on vote totals -- though neither will be mathematically able to earn enough support in that category to capture the nomination prior to the party convention in late August.
Former President Bill Clinton, Chelsea Clinton and other campaign surrogates have been calling and visiting superdelegates, both uncommitted and previously committed Clinton backers who seem to be wavering in their support. Watch Bill Clinton's statements on his wife's campaign »
Both sides have kept up a steady stream of near-daily calls to those who remain undeclared.
"Nothing like being in Costco and getting a call from a presidential candidate," joked Klobuchar Monday, without saying which campaign she was referring to.
The New York senator herself has begun to speak candidly about the calls for her exit -- and her intention to stay in the race through the convention.
"There are some folks saying we ought to stop these elections. I didn't think we believed that in America," she said in Indiana this weekend. "I thought we, of all people, knew how important it was to give everyone a chance to have their voices heard and their votes counted."
Clinton herself has made several comments in recent weeks highlighting the fact that most pledged delegates are free to switch sides under party rules. But that scenario is unlikely -- if Clinton is to win the nomination, it is critical for her to maintain and build on her diminishing superdelegate support.
And many undeclared superdelegates, particularly those in red states and swing states where the outcome of lower-tier races are in doubt, have said publicly they are attracted to Obama's "coattails" -- his ability to attract new voters in traditionally red areas -- that they say could help Democratic candidates running for any office.
More troubling for Clinton, party leaders have started to fret publicly that the Democratic primary contest has grown so bitter that it may begin to pay dividends for Republican Sen. John McCain's campaign.
Both Casey and Klobuchar said this factor helped them decide to back Obama now, despite earlier plans to remain neutral through the primary season.
The clock is ticking for the remaining holdouts: Last week, Democratic National Committee Chairman Howard Dean urged all undeclared superdelegates to weigh in before July 1.
Yet Dean, like most recent endorsers, has been reluctant to urge Clinton to end her run. Watch Howard Dean's discuss the Democratic race »
"She should continue her campaign," said Klobuchar on an Obama campaign conference call announcing her decision to back his candidacy. "I have faith that our candidates will figure this out and this will come to a conclusion in early summer. ... They are strong people and have good hearts."
Last week, Vermont Sen. Pat Leahy, an Obama supporter, said bluntly the time had come for Clinton to bow out and back Obama.
Obama has resisted joining the chorus, telling reporters this weekend: "My attitude is that Sen. Clinton can run as long as she wants." E-mail to a friend